The present exemplary embodiment relates to document processing systems such as printers, copiers, multi-function devices, etc., and more particularly to mitigation of side 1 to side 2 process magnification errors (sometimes referred to as “show-thru” or “see-thru error) in printing engines and duplex printing methods for printing images on two sides of a printed substrate. This form of image-on-paper (IOP) magnification error is seen as a difference in the image size for images printed on two different sides of a printed substrate, and is unacceptable in many customer applications. Show-thru errors are mainly caused by the substrate shrinking when fed through a fusing station after the image is transferred to the first side of the substrate and before an image is transferred to the second side. In particular, conventional duplex printing systems include a duplex routing station and media inverter downstream of the fusing station that reintroduce a printed sheet into the transfer path before the once-printed sheet has had time to be reacclimated to the ambient temperature. The side 2 image is then transferred to the shrunken paper and becomes larger relative to the side 1 image once the paper resumes the original size. This results in a process magnification error evident as a show-thru discrepancy between the image sizes on either side of the substrate, with the side 2 image appearing larger than the side 1 image in the process direction.
The error can be addressed somewhat by shifting the margin for the side 2 image in order to evenly distribute the magnification error equally on both sides. However, this approach does not reduce the process magnification error, but instead makes it less apparent upon visual inspection.
Image data may be manipulated to artificially shrink the side 2 image, for instance, by removing certain data, but this leads to image defects.
Another technique to address side 1 to side 2 IOP magnification is described in U.S. Patent Application Publication No. 2010/0020364, published Jan. 28, 2010 and filed as Ser. No. 12/177,376 on Jul. 22, 2008, and assigned to the assignee of the present disclosure, the entirety of which is hereby incorporated by reference as if full set forth herein. That approach applies a different image magnification to side 2 that is used to compensate for paper shrinkage due to moisture lost from a fuser and other sources by implementing a method to change raster output scanner (ROS) motor polygon assembly (MPA) speed when imaging side 2 of a sheet compared to side 1.
The entirety of U.S. patent application Ser. No. 12/645,603, filed Dec. 23, 2009 and assigned to the assignee of the present disclosure, is hereby incorporated by reference as if full set forth herein. This application discloses techniques and apparatus for controlling the ROS motor polygon assembly (MPA) control frequency using a numerically controlled oscillator (NCO) to generate a clock frequency. The NCO output is used to source a square wave digital clock signal that is, in turn, used as an input to a ROS Motor Polygon Assembly (MPA) velocity control circuit. The ROS MPA control circuit regulates its speed by phase locking to this NCO generated ROS Master Clock (RMC) signal.
Other approaches for addressing side 1 to side 2 IOP magnification employ data modification, such as Contone High Resolution Image Path Electronic Registration (CHIPER), as shown in U.S. Patent Application Publication No. 2010/0309526, published Dec. 9, 2010 and filed as Ser. No. 12/480,945 on Jun. 9, 2009, and assigned to the assignee of the present disclosure, the entirety of which is hereby incorporated by reference as if full set forth herein. This technique compensates for the misregistration error caused by fuser shrinkage/stretching by adding/removing pixels or subpixels to/from the image prior to printing.
However, these techniques do not provide a complete solution, particularly in advance printing systems operated at high speeds. Consequently, a need remains for improved printing systems and duplex printing techniques by which the adverse effects of side 1 to side 2 IOP magnification errors can be mitigated and image quality performance can be improved.